It is my long-term goal to determine the mechanisms of axon guidance that allow axons to make precise decisions when encountering repellent cues. By studying these mechanisms, it is my hope that critical insight is gained to begin to treat patients with spinal cord injury, by reestablishing the signaling that would promote proper axon guidance and growth for axons to overcome the repulsive environment of the spinal cord. I propose to study one particular axon pathway in the spinal cord - the commissural (C) axon pathway. C axons integrate sensory information from peripheral nerves and extend into the brain for high level processing. Therefore, it is important to understand the mechanisms by which C axons are guided within the spinal cord, in order to effectively design treatments that will help to reestablish these axon pathways in the case of spinal cord injury. My specific aims are to 1) Characterize the role of BMP signaling in axon outgrowth, and 2) Determine the downstream mechanism by which BMPs regulate axon outgrowth. For specific aim 1, I will be extending upon an unexpected finding I made in my most recent publication (Yamauchi et al., Development 2008): activation of BMP signaling in C axons causes a stall in outgrowth. By introducing an activated form of the receptor I identified as mediating BMP chemorepellent signaling (BMPRIB) into chick spinal cords, I observed an in vivo delay in outgrowth that is unlikely due to a failure in BMP chemorepulsion. Therefore, I propose to characterize this delay in outgrowth by an in vivo time course of the rate of C axon extension through the spinal cord while expressing constitutively active BMPRIB. I will further test whether BMPRIB signaling negatively regulates outgrowth by observing a time course of axon extension in C neurons from BMPRIB null mice. For specific aim 2, I will investigate whether a specific kinase, LIMK1, is the downstream effector mediating the stall in axon outgrowth, as is suggested by our preliminary findings. I will examine a time course of LIMK1 phosphorylation (activation) elicited by BMP stimulation in dissociated C neuron cultures. Then, I will examine whether BMPRIB is required for LIMK1 activation by stimulating C neurons in culture that are BMPRIB deficient. Lastly, I will examine whether the actin severing protein cofilin, which is the only known substrate for LIMK1, is the rate limiting protein through which BMP signaling negatively regulates axon outgrowth, by overexpressing it in vivo to rescue BMP-mediated stalled outgrowth. This proposal will establish a mechanism by which BMP chemorepellent signaling inhibits axon outgrowth, which may lead to therapies aimed at relieving the inhibitory environment of the CMS in spinal cord injuries.